1. Field of the Invention
This invention relates generally to the field of semiconductor memory devices, and more particularly to nonvolatile semiconductor memory devices of the type known as insulated gate field effect transistors (IGFETS).
2. Description of the Prior Art
In the semiconductor arts, nonvolatile semiconductor memory devices including MIS (metal insulator semiconductors) and MIOS (metal insulator oxide semiconductor) structures are widely used. Typical examples of these types of storage devices in the nonvolatile semiconductor memories have MNOS (metal nitride oxide semiconductor) structures. The structure takes the form of a silicon substrate having a source and drain formed therein separated by a channel having a gate spaced between the source and drain and spaced above the channel by a combined silicon dioxide-silicon nitride layer. Carriers such as electrons injected through the insulator layer from the substrate are trapped near the interface of the silicon dioxide and silicon nitride layers. The carriers trapped at the interface will be retained nonvolatilely for several years or more.
In the above mentioned MNOS device the thickness of the silicon dioxide layer is about 20 angstroms and the layer overlaps the source and drain junctions, so the avalanche breakdown voltage of the drain relative to the substrate is lowered. Moreover the breakdown voltage between the source (or drain) and gate is lowered.
To solve this problem an improved MNOS device is disclosed in an article entitled "The drain source protected MNOS memory device and memory endurance" published 1973 IEDM pp. 126-128. Here a thicker gate oxide layer overlaps the source and drain, thus the breakdown voltage between the source (or drain) and gate is improved. However in this device, which exhibits a bivalued threshold voltage, the voltage difference between the low and high threshold values is small, causing the error in reading out the stored threshold value.